kangaroo court

A self-appointed tribunal that violates established legal procedure; also, a dishonest or incompetent court of law. For example, The rebels set up a kangaroo court and condemned the prisoners to summary execution, or That judge runs a kangaroo court—he tells rape victims they should have been more careful. This expression is thought to liken the jumping ability of kangaroos to a court that jumps to conclusions on an invalid basis. [Mid-1800s]